In accordance with an increased interest in safety of cars in recent years, requests for not only a low fuel consumption but also a steering stability have been increased. In order to meet the requests, tires highly satisfying low heat build-up achieved by reducing rolling resistance, a steering stability on wet road surfaces and dry road surfaces, and abrasion resistance in tire performances have been required. Also, in order to meet such requests as described above, reinforcing fillers and rubber components are tried to be improved.
Methods of (a) increasing an amount of a filler, (b) raising a grade of a filler (atomization, highly structuralizing), and (c) lowering a glass transition temperature Tg of a polymer used (increasing a use proportion of natural rubber NR and butadiene rubber BR) are usually carried out in order to improve the abrasion resistance. However, as for the performances of a tire are concerned, the rolling resistance is deteriorated or the abrasion resistance is improved at the sacrifice of the wet grip performance by the methods described above.
If a filler is increased in a blending amount or raised to a higher grade, not only the rolling resistance is deteriorated, but also the workability in refining the rubber gets worse, and they have to be improved. Accordingly, the oil amount is increased in many cases, but this method is followed by deterioration in the rolling resistance and reduction in the frictionality, and the targeted effect of improving the abrasion resistance is limited.
Carbon blacks are used as a filler for reinforcing. This is attributable to that carbon black can provide a rubber composition with high abrasion resistance. However, it is difficult to obtain by using singly carbon black a rubber composition in which a wet grip performance, abrasion resistance and a low fuel consumption are balanced at a high level, and silica is compounded instead of carbon black as an improving method therefor. However, it has been found that when silica is used as a filler, a blending amount of carbon black is relatively reduced, so that the rubber composition is inevitably reduced in fracture strength and abrasion resistance. Also, involved therein is the problem that silica has inferior dispersibility in rubber and that a rubber composition is increased in a Mooney viscosity in kneading and inferior in processability such as extrusion.
Rubber compositions for tire tread containing rubber components and inorganic compound powders in addition to carbon black and silica as fillers for the purpose of obtaining rubber compositions which are excellent in a low fuel consumption and a wet grip performance without lowering abrasion resistance are proposed in patent documents 1 to 3.
However, in the rubber compositions described in the patent documents 1 to 3, the inorganic compound powders have to be compounded in a relatively large amount in order to obtain a satisfactory effect of improving a wet grip performance and a fuel consumption. In the case, the abrasion resistance tends to be readily reduced, and therefore a rubber composition for tire which is further excellent in a balance between a wet grip performance, abrasion resistance and a low fuel consumption has not been obtained.
When a polymer having a low glass transition temperature is used, the abrasion resistance and the rolling resistance are improved at the same time, but on the other hand, the problem that the wet grip performance is reduced to a large extent is involved therein. A blending amount of silica is usually increased in order to solve the problem, and as a result, not only the rolling resistance but also refining and extruding works are deteriorated. If the blending amount is controlled to an optimum amount in a range in which the productivity is not damaged, the abrasion resistance and the rolling resistance are improved only to a small extent and stay unchanged from the existing ones.
Proposed in patent document 4 is a rubber composition for tire tread which is improved in a wet grip performance and abrasion resistance by using carbon black specifying a characteristic calculated from two-dimensional projection image analysis of aggregates and properties such as DBP and N2SA in addition to a diene base rubber component containing 70% by mass or more of a styrene-butadiene rubber having a glass transition temperature of −35° C. or higher, and silica. However, it does not sufficiently satisfy a low fuel consumption in addition to a wet grip performance and abrasion resistance.
Also, the present applicants filed previously a rubber composition for tire prepared by using novel carbon black used in the present invention in Japan (Patent Application 2013-097622).